1. Field of the Invention
This invention pertains in general to the field of vaporizing devices and methods for breathing apparatuses, in particular anesthetic vaporizers. More particularly the invention relates to an anesthesia injection vaporizer device and method for vaporizing a liquid anesthetic agent in a breathing apparatus.
2. Description of the Prior Art
German published patent application number DE4105370 of Drägerwerk AG, published in 1992, discloses an anesthetic vaporizer device. The vaporizer is of the evaporation vaporizer type, as disclosed in German published patent application number DE1271903. A liquid anesthetic agent is stored in a vessel. The liquid anesthetic agent is drawn up by a cylindrical wick. Air becomes saturated with the anesthetic agent by flowing past the wick. The liquid is kept at a constant low temperature by a latent heat storage device to avoid unintentional evaporation. This is in particular applicable to liquids having low evaporation temperatures around room temperature, such as Desflurane. The latent heat storage device disclosed in DE4105370 is arranged in the evaporation vaporizer to keep the temperature of the anesthetic vaporizer device constant, namely at a temperature lower than the evaporation temperature of the anesthetic agent.
In more detail, the anesthetic vaporizer disclosed in DE4105370 device consists of an outer vessel with double walls and with the space between the walls being filled with thermal insulation. The outer vessel is filled with a material, such as a wax, which melts at the same temperature as the required storage temperature of the liquid anesthetic agent. It should be observed that a storage temperature for an anesthetic agent liquid is substantially lower than its boiling point or evaporation temperature. Any change in ambient temperature causes the wax to melt or solidify so that the wax gains or releases latent heat without any change in the temperature of the wax. The wax is thus merely arranged as a heat buffer, improving isolation of a large volume of the liquid anesthetic agent inside a container of the evaporation vaporizer in relation to temperature changes of the environment surrounding the evaporation vaporizer.
Since 1992 development of anesthetic vaporizers has advanced considerably. Injection principle based vaporizers have been developed, and the devices are commonly known as injection vaporizers.
In a modern injection vaporizer injecting a liquid anesthetic agent for evaporation purposes, requirements are very high regarding the quickness of the vaporization process. An injection vaporizer injects or sprays intermittently a pulse of a small amount of the anesthetic agent liquid into a vaporizing chamber or a channel by means of an atomizing technique. A spray of droplets of the liquid is generated by the injection. The liquid is thus gasified into a flow of a carrier gas, often a mixture of nitrous oxide or air and oxygen. The mean flow of the liquid is controlled by the time length of a pulse and the frequency of pulses. Typically, such dosing pulses have a duration between 2 to 10 msec.
The vaporization rate of the droplets in turn depends on the amount of heat energy available for providing the transformation from the liquid phase to the gaseous phase of the anesthetic agent. It depends further on the removal rate of the gaseous medium from the surface of the liquid droplet to keep the vaporizing process ongoing until the droplet is completely gasified.
Known injection vaporizers are based on a temperature regulation of the temperature of the vaporization chamber that only control the mean temperature therein in a slow process. Temperature regulation can thus not follow the quick injection process and related vaporization events following the aforementioned extremely short injection pulses.
The power that would be needed to provide the heat energy necessary to instantly gasify injected liquid is in the range of approximately 60 to 80 Watts. However, such high power is not available at the vaporization site. In addition, breathing apparatuses in which the injection vaporizers are used, have limited effect available due to safety requirements concerning battery backup drift, limiting available power.
Moreover, temperature regulation is slow as feedback is thermistor based. The thermistor has a time constant of several seconds and temperature regulation thus in the range of Hz. This allows only for a correspondingly slow temperature regulation process of the mean temperature in the vaporization chamber.
Vaporization in an injector vaporizer thus takes place in two stages. A minor portion of the liquid gasifies during the flight from the injector towards an interior wall of a vaporizer chamber in the injection vaporizer unit. The major portion of the injected liquid is gasified in a secondary stage from the wall of the vaporizer chamber.
However, due to a low thermal capacity of the volume available for the vaporization process, temperature drops quickly in the vaporization chamber following the injection pulse. This temperature drop may lead to such low temperatures that the vaporization of the liquid injected anesthetic agent ceases. This would be undesired in clinical operation
The latent heat storage device of DE4105370 is arranged to keep the entire vaporizer device at a constant temperature below the vaporization temperature and is furthermore not suitable to compensate for such quick, locally occurring temperature changes inside a vaporizing chamber of modern injection vaporizers.
Thus, there is a need for an improved injection vaporizer. The improved injection vaporizer should advantageously provide effective vaporization.